Containers for beverages such as milk, fruit juices, and drinks are conventionally constructed from blanks of thermoplastic coated paperboard. The most widely used of such containers have a rectangular cross-sectional body surmounted by a gable-top closure incorporating an extensible pouring spout. Blanks from which the containers are constructed are divided into a plurality of panels that are adapted to form the walls and closure members. The panels are formed and separated by score lines at which the blank is folded. Particular panels are intended to be joined together in a lapped arrangement in the completed container. Typically those panels are pressed together and heated, or exposed to high frequency radiation to fuse the adjoining thermoplastic surfaces and form a seal. To seal the top of a filled container, two or more panels are joined and sealed to form a rib along the top edge of the roof panels. Exemplary of such container blanks are those shown in U.S. Pat. Nos. 2,750,095 (Alden) and 3,245,603 (Wilcox).
Most gable-top containers have certain characteristics in common. Four side panels are connected together along their respective longitudinal edges, and a set of bottom closure panels cooperate to seal the bottom of the container. The top of the container consists of two roof panels and two triangular end panels, each of which is connected to the top edges of the side panels. Other pouring spout panels connect the roof panels and the triangular end panels, and include first and second foldback panels and first and second roof wing panels. Attached to the uppermost edge of each of the roof panels is a roof rib panel, and an upper rib panel is connected to the uppermost edge of each roof rib panel. A gable rib panel is connected to the upper edge of each of the first and second foldback panels, and the center of the gable rib panel typically forms a pouring spout tip. The pouring spout panels, which comprise at least the triangular end panel, the first and second foldback panels, and the first and second roof wing panels, are adapted to shift between a closed position and an open position. When in the closed position, the gable rib panel is located between the roof rib panels, and is sealed thereto during the sealing process. When in the open position, the pouring spout panels are extended to allow dispensation of the contents of the container. It should be noted that most gable-top containers comprise two pouring spouts, although only one is typically used for dispensing the contents of the container.
Containers of this type are opened for access to the contents by a two step toggle action process. First, the edges of the roof panels at the front of the container are pushed outward and upward toward the rear of the container by thumb pressure, breaking the seal in the roof rib and upper rib panels surmounting the roof above the pouring spout. This will be referred to as foldback, or Stage I opening. Second, the edges of the roof panels are pushed forward and toward each other. The forces are communicated through spout panels to the tip of the pouring spout, breaking the seal between the gable rib panels and the underside of the roof rib panels and snapping the spout outward to a pouring position. This will be referred to as extension, or Stage II opening. For purposes of this invention, Stage II opening forces are the more critical, because the seal at and near the pouring spout tip is completely separated during Stage II opening
It is difficult to form consistently a gable-top container that is both adequately sealed and easy to open. The difficulty is due in large measure to the circumstances attendant the sealing of the container. When the pouring spout is folded into a closed position, the container panels comprising the edge of the pouring spout are directly adjacent each other, resulting in an area of double thickness cardstock. However, in the area proximate the tip of the pouring spout, the cardstock has been deformed by buckling and its thickness is greater than a double thickness of cardstock. For example, in a conventional gable-top container with the spout in a closed condition, the area of double thickness cardstock adjacent the pouring spout tip has been measured at 0.050" thick. The pouring spout tip itself has been measured at 0.071" thick, an increase of 42% due to buckling in the tip region.
When conventional sealing means are applied to the upper panels of the carton, planar sealing jaws apply a force against the roof rib and upper rib panels that is equally distributed over the area of each jaw. However, because the area proximate the pouring spout tips is thicker than the remainder of the roof rib and upper rib panels, the application of sealing force results in higher pressure in the area surrounding the pouring spout tips. The result is a pinching action in the area proximate the pouring spout tips, which renders the carton more difficult to open. Furthermore, the increased thickness in the center of the carton top is compressed during sealing, and when the sealing pressure is released the panels have a tendency to spring back into position, which pulls the seal apart. This problem is particularly prevalent in cartons incorporating a stiffening fillet on one or more of the pouring spout panels, as shown in U.S. Pat. Nos. 4,726,234; 4,813,548; 4,872,562; 4,756,426; 4,792,048; 4,869,372; 4,712,727; 4,813,547; and 4,869,373.
One possible solution to the problem presented would be to reduce the overall force applied to the sealing jaws. While this would certainly reduce the pressure in the area proximate the pouring spout tips, the pressure reduction would compromise seal integrity in the remainder of the upper panels. Abhesives have found some application in gable-top container sealing, and the pouring spout tips could also be coated with such an abhesive to prevent or reduce effective sealing in designated areas. However, abhesives are typically very temperature sensitive, and because sealing temperatures cannot always be accurately controlled, abhesives are also an unacceptable solution.
Thus, it is desirable to provide a gable-top container having a reduced opening force, while not allowing carton spring back to separate the seal, or damaging the pouring spout in any way. This problem is not adequately solved by any of the conventional gable-top container designs, and in particular with gable-top container constructed from higher strength laminates.